The watery entrance for condemned prisoners heading to the Tower of London is still visible along the Thames.

Hear the quietest sound from across the dome.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

The most iconic structure in Scotland has been home to bishops, colonels, and Sean Connery characters.

These fantastically named natural pools look like something out of a Scottish myth.

One of Edinburgh's oldest pubs hides a 19th-century skittles alley in the back.

The most loyal of little dogs, or a Victorian era publicity stunt?

This bridge was constructed from the rubble of the infamous Bastille.

This former “ante-chamber to the guillotine” has been refurbished to illustrate the brutal history of French justice.

A memorial to the nearly 10,000 American soldiers laid to rest on the beaches of Normandy during WWII.

A sacred site for Native Americans surveyed by George Washington and owned by both King George III and Thomas Jefferson.

The colonial city's social history is reflected in its fanciful door knockers.

Sloth rehabilitation center in the jungle of Costa Rica.

A magical sinkhole on the Yucatán Peninsula is adorned with mini waterfalls and hanging vines.

Every equinox this Maya pyramid puts on a spooky ancient light show.

No telescopes, no power, no gift shop: The Observatory of Chichen Itza, one of the world's oldest observatories, takes astronomy back to basics.

These roaring primates were considered a divine creature in the cosmology of the Maya.

Monkeys inhabit the lush forest surrounding ancient Mayan ruins once considered sacred.

Sparkling, calcite-covered skeletons of children who may have been sacrificed to appease an angry god.

This portion of a lovely Belize beach is infested with (harmless) sharks and rays.

A king's pet, a pope's gift, an artist's inspiration, and a tragic shipwreck.

Elaborately carved to showcase the glorious wealth that the age of exploration brought to the Portuguese empire.

Legacy of the earthquake that nearly wiped Lisbon off the face of the Earth.

This unreal Portuguese palace looks as though it's made from a pile of different castles.